JPH06205221A - Method of manufacturing halftone image - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a method for producing a halftone image having a dot pattern that can be used to reduce the effects of rosette moiré, dot gain and tone jumping in the halftone image. A halftone image with dot patterns can be created by printing dots with inwardly curved edges (Fig. 4). In light tones, these dots can resemble a needlepoint with an exaggerated point. In the dark tones, these dots effectively merge to create unprinted dots that can form an oval shape.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to printing methods and, more particularly, to dot patterns used in producing halftone images. These patterns include some moire effect perceptibility and shapes that reduce the jumps of the various tones that often occur.

[0002]

BACKGROUND OF THE INVENTION In most printing methods, only one of each available ink color is printed on a print medium.
I could only give one tone. Tone changes are obtained by breaking up each image into fine dots of various sizes on a grid or screen grid. The color change is usually obtained by overlaying screens with the primary colors cyan, magenta and yellow plus black for boundary definition. Ideally, human vision integrates well-created image-distributed dots into the exact impression of the original screen. The final image almost always contains many compromises between practical limitations and shortcomings of the printing method and what can actually be perceived by the human eye and brain.

Multiple round dots made at 45 ° to the vertical are perceptible at least for a given grid spacing or screen line, and a single color image is usually printed in this way. . Other dot shapes, such as square and elliptical (diamond-shaped) shapes, or combinations thereof are sometimes used, but because they all generally have straight or outwardly curved edges along their entire length. is there. These dots are typically arranged on a square grid with rows spaced about 30 / cm for newsprints and about 60 / cm for high quality images. Rectangular or other lattices with low symmetry are also sometimes used. In light tones, the dots remain distinct on the light background provided by the print medium, but merge into the dark tones to appear as light dots on a dark background. Thus, the printed and unprinted areas of an image are each inverted from dot to background and vice versa as the tone darkens.

When printing color images, large-scale patterns and small-scale patterns often appear due to periodic dot alignments throughout and at their edges. Large scale patterns are usually bands that intersect on the order of 10 or more dots along each side to form a square. Such effects are substantially eliminated by proper relative rotation of the color screen from horizontal to counterclockwise, such as 15 ° for cyan, 45 ° for magenta, 90 ° for yellow and 75 ° for black. . Often complex mathematical procedures are performed to determine the proper angle. Placing the screen with no rotation or misalignment causes a color shift in the image, especially as colors of different opacity in black and yellow are consistently overlapped over a large range. is there.
In any case, subtle color shifts occur due to screen misalignment during overlay. Small moire effect scale patterns are usually rosettes that are on the order of a few dots wide, and such rosettes produce perceptible speckles in areas of uniform color. This effect has proven to be even more difficult to eliminate.

In halftone printing, inaccuracies in the reproduction, known as dot gain, often cause yet another problem between tones that become larger as adjacent dots are connected. is there. For example, in lithography, including offset printing, a grease-like ink is fixed to the printed area of the image by wetting the surrounding unprinted area with water. Unfortunately, the surface tension of the ink / water causes or improves the connection between parts of the print area that are in close proximity, resulting in a sharp tone jump. The absorption of ink on poor quality print paper also results in such linkages. The round and square dots on the square grid naturally meet the nearest neighbors with a print areal density of 78% and 50%, respectively. Dot gain causes concatenation at slightly lower densities and slightly higher densities to enhance such concatenation, resulting in discontinuities in the intended range of smoothly varying tones. Such effects are also difficult to completely eliminate.

The production of halftone images is mostly done using computer controlled equipment such as scanners and imagesetters. Reproduced photographs or other technical items are scanned and the original scene is stored in electronic memory or directly massed. The image is manipulated and / or combined with text before a print medium such as a film or plate is made. The image is usually converted to a halftone dot screen only in the final stage. Such operations are complex software operations that can be tailored to suit a particular image. Similar software can be used for desktop publishing work.
ing) and other electronic printing and imaging methods. Precise control of the dot pattern is necessary to produce an acceptable image, and the computer work required for high quality images is often extensive and time consuming. For example, a software method for reducing the moire effect is US Pat. No. 4,084,183 (congruent screen).
en), U.S. Pat. No. 4,894,726 (Quasi Periodic Screen) (quasi period)
ic screen), European Publication 37027
1 (elongated ordinary dot), wo 90/10991
(Linear screen transition) (rectilineear
screen transfer) and w
90/86034 (pseudo random change in dot shape). Some images are still manufactured using conventional opto-mechanical devices such as contact screens. A well-known summary of the known dot pattern, its various problems and moire effects can be found in Color Screening Technology, a tutorial in the basic version (T
U.S.A., Seybold Report on Desktop Publishing, Volume 6, Issue 2, October 1991, Seybold Publications Incorporated, Pennsylvania, USA, and Desktop Two Press, Ninth Edition, February 1992. Issue, Peter Fink Communications, Inc.,
Can be found in California, USA.

[0007]

It is an object of the present invention to provide an alternative dot pattern that can be used to reduce rosette moiré, dot gain and tone jumping effects in halftone images.

[0008]

A halftone image having a dot pattern according to the present invention can be produced by printing dots having inwardly curved edges. In light tones, these dots may resemble a needle-pointer with exaggerated cusps. In the dark tones, these dots effectively merge to create unprinted dots that can form an oval shape. The tone is "bright" to "dark"
As it changes to, the needle-like object gradually becomes elongate along one direction and becomes two separate steps, the first step in the direction of extension and the second step in the direction substantially perpendicular to it. It will merge into the nearest adjacent part.
Dot screens are usually formed by a square grid, thus resulting in the four most adjacent portions which are normally symmetrically arranged at equal intervals. As the tones continue to darken after they merge, this stretching effect diminishes, thus causing oval unprinted dots to come closer to the dot. Such a pattern can also be thought of as an inverted elliptical dot pattern in which the overall ellipse changes from a round shape to a maximum ellipticity and returns to a round shape across the full range of tones. When many screens are overlaid, the inwardly curved edges of the printed area do not align, thus easily forming a perceptible rosette similar to a regular dot. Furthermore, this elongation effect
It can be varied so that the dot gain is created within the tone such that the perceptibility is also minimized.

As of the time this specification has been written, the best use of the invention and its results have not been fully explored. When producing a color image, the primary color and the black screen are 45 to each other.
It is best placed at a separation of °. For example, cyan 45 °, magenta 135 °, yellow 90 °, and black 45 °. Screens separated by 90 ° can generally be considered to be placed at 0 °, unlike the above, so that in this example there are essentially only two angles for the calculation of the dot pattern, namely 0 ° and 45.
There is only °. This represents a significant computational simplification over normal screen angles.

Cyan and magenta screens usually have a well considered misregistration or misalignment with each other to avoid the risk of color shift. The yellow and black screens can also be offset from each other and printed with dots that are only slightly stretched or unstretched. The yellow and / or black lattice spacing can also be increased or decreased relative to cyan and magenta. The final determination of these possibilities has to wait for the implementation of the complete software of the invention and is relevant to the particular image.

[0011]

An example of the invention is explained below with reference to the accompanying drawings.

Referring to these figures, for purposes of illustration, the dot pattern is approximately 1 in black and white.
It can be seen that it has been expanded from 0 to 100 times. The visual effects resulting from the human eye's integration of halftone images on a normal scale are not clear, but must be recognized by the skilled reader. In particular, the improvements seen in the color image due to the inward curvature of the edges of the printed dots from "bright" to the midtones, and those obtained in the single color image due to dot stretching are seen.

The various forms of computer hardware and software used to implement the dot patterns according to the present invention will also be apparent to the skilled reader, or at least obtained in light of the above-referenced items. is there. For example, publishing work on a desk (de
A range of sktop publishing and high end scanner devices and software are available from suppliers such as Adobe, Agfa, Crossfield, Rhinotype Hell and Hite and Scitex. As new patterns are developed, hardware and / or software can be correspondingly upgraded. A conventional method of producing a halftone image, particularly a color image, as implemented on the apparatus is shown schematically in the flow chart of FIG.

Generally speaking, a scene is electronically scanned by film or other printed technology and the resulting data is stored as pixel-based color and intensity information. These pixels are generally aligned in the vertical and horizontal directions of scanner motion. These data are then processed into a standardized format, commonly known as the trademark PostScript (Postscript), which is then made to four halftone screens showing primary color and black on demand. . These screens are made from pixel information by various raster image processor programs that calculate dot shape, grid spacing and screen rotation angle. The actuator usually has a range of dot patterns obtained by software built into the device. The actuator selects the appropriate shape, spacing and angle until it obtains an acceptable image during proofing. In a conventional printing operation, each screen is individually output and made into four films or plates, which are used to print multiple copies of the final black and white or color image. Of.
This part of the method is used as an image setting in the case of desk-top publishing work, and also as a "high-end" system.
d system), known as scanner output. In other operations, such as inkjet printing, it is possible to output the color image directly.

FIGS. 1 (a), 1 (b) and 1 (c) show smoothing from a printed area density of about 10% for light tones to an area density of about 90% for dark tones. 3 shows regular dot patterns of circles, squares and "ovals", respectively, on a square grid that changes to. Round dots have bounding outwardly curved edges, while square or diamond shaped dots have flat edges between the four vertices. The square pattern is reversible in that the printed area in light tones has the same shape as the unprinted area in dark tones.

Although FIG. 2 shows a more complex dot pattern, in this dot pattern the normal round dots in light tones are squared by the inversion of the round dots in dark tones in the middle tones. This is often referred to as the Euclidean dot pattern. The curvature of the edges of the printed area changes correspondingly from the outside to the flat and inward, respectively.
The Moire effect does not occur in FIGS. 1 and 2. Because there is only one screen in each case. Not obvious because the dot gain is magnified.

FIG. 3 illustrates an inverted round dot pattern that uniformly changes from "bright" to "dark" tones.
Dots in light tones have an inwardly curved edge between the cusps distributed in two planes that are mirror image symmetric. The point may possibly be flat or rounded if desired. Each dot merges into the four nearest neighbors at the same time with an area density of approximately 22%. Otherwise, the inward curvature extends along the entire length of substantially all printed area. The non-printed areas have corresponding outwardly curved edges and are separated to appear as round dots in dark tones. Overall, this pattern effectively appears to be the inverse or negative state of the normal round dot pattern in FIG. 1 (a) over the entire range of tones, in this respect on a dark printed background. Can be thought of as a pattern of bright round dots. Due to such inversion, the edges of all printed area portions are circular or arcuate, but may be joined by other smooth curves or edge points if desired. Also, the dots need not be quadruple symmetrical or printed on a square grid as shown. The rosette moire effect in a color image can be reduced by a printed dot pattern with inwardly curved edges, as will be apparent from the figures shown after the two grids are superimposed. However, the jumping of tones due to dot gain still leaves a problem as in a regular round dot pattern.

FIGS. 4a and 4b show an inverted oval dot pattern according to the present invention. In this form, the generally needle-shaped dots are stretched along a mirror image symmetric plane from the light quarter to the midtone, with the nearest neighbor along the direction of darkening of the tone first. Vertical merging, and then merging in a horizontal direction at a right angle. Each dot is about 22% and 3
It joins two opposing pairs in two separate steps with an area density of 6%. This pattern effectively appears to be the inverse or negative state of the round dot pattern where the dots become oval in the right-angle tone. Figure 4 (a) shows 0%
4b shows the full range of tones from 1 to 100%, and FIG. 4 (b) is a more detailed view of quadruple / duplex or different circular / oval changes with an areal density of about 15% to 39% Is shown. The rosette moiré effect is reduced as in the pattern of Figure 3, but in this case the perceptibility of any tone jump is controlled by controlling the dot stretching action to bring the dot gain into two less distinct stages. It can be reduced by splitting and allowing this to also be moved between a range of tones.

5 (a), 5 (b) and 5 (c) show examples of edge variations of the dot according to the invention superimposed on a common center. Diagonal squares facilitate comparison and measurement during manufacturing. These edges do not necessarily have to be mathematically elliptical, but can be any suitable oval or roughly similar shape that can be computer generated. 5 (a) is a gentle circle of FIG. 4 (a) and FIG. 4 (b) -oval-
The change in circle is shown. 5 (b) and 5 (c) show the greater distortion caused by the greater separation of the dot gain stages.

6 (a) and 6 (b) show the reduction of the rosette moire effect obtained according to the invention. FIG. 6 (a) shows a rosette made by superimposing two conventional round dot screens at 30 ° as is commonly done for cyan and magenta in color images. FIG. 6b shows a smoother variation of a similar tone made by superimposing two inverted variable oval dot screens at 90 ° according to the present invention. Producing a color image using all four screens is not as straightforward.

The primary color screen is cyan 45 °,
45 ° like magenta 135 ° and yellow 90 °
It has been found that it is best to place the blacks at 0 ° magenta and 90 °, with the blacks being spaced apart. Moreover, not all screens need necessarily be printed according to the present invention. For example, at least cyan and magenta should use elongate dots in an inverted variable oval dot pattern, while yellow can simply use an inverted round dot pattern. The black can be an inverted round dot pattern, or the normal round dot pattern itself can be used to give a pleasing image.

It has also been found that it is often not necessary to produce a well-formed color image.
For example, the primary color screen could be used alone with the above pattern at various angles.

It is also generally necessary that the cyan and magenta screens be offset from each other to avoid color shifting. If these screens are formed by a square grid, then to get the best result considering the misalignment due to accidents that often occur during printing, this position should be offset in the direction of the grid. In any case, it should be about half of the lattice spacing in parallel. The yellow and black screens also sometimes have to be offset to avoid color shifts. The effect of displacing these positions has to be explored further, but is obvious to the skilled reader working on a particular image.

In some images, the potential for color shifting and moire effects was described above by reducing the black lattice spacing by a factor of about cos 45 ° (about 0.71) compared to the cyan and magenta lattice spacings. It is reduced by the 45 ° angle and the inverted pattern.
In the few cases, the yellow grid spacing was correspondingly increased by this factor. Here again, the effect of these adjustments on the lattice spacing has to be explored further, but will be clear to the skilled reader working on a particular image.

FIGS. 7 (a) to 7 (i) show dot patterns of tests conducted to date according to the present invention. From FIG. 7A to FIG. 7D, cyan 135 is used.
°, magenta 45 °, yellow 0 ° and black 0 /
Shown are individual 90 ° screens, where at all tone values the printed and unprinted areas were joined at two or four of their nearest neighbors. Cyan and magenta used a variable oval dot pattern that was inverted, and yellow and black used a round dot pattern that was simply inverted. The black lattice spacing was reduced by a factor of about 0.71 relative to the others. Figures 7e to 7g show screens superimposed in pairs, i.e. cyan / yellow, magenta / yellow and cyan / magenta pairs. In (f) and (g) of FIG. 7, cyan and magenta were displaced by half the lattice spacing in parallel to the direction of the lattice, which is a diagonal line on the paper surface as shown. The visual effect is somewhat unrealistic in that yellow is usually substantially less eye irritating than the darker colors, in which case all colors must be shown the same in black. FIG. 7H shows superimposed cyan, magenta and yellow screens. Only a slight moire effect is apparent in nature. This is because these colors must be shown in black as mentioned above. 7 (i) is superimposed on FIG. 7 (h), and FIG. 7 (d) is superimposed.
Shows a black screen of. Here again there is a slight moire effect due to the overall black color illustration, but under normal printing conditions the colors have different stimuli and the final image screen is virtually moire-free. is there.

8 (a) to 8 (i) are shown in FIG.
7A shows the result of an attempt to compare with (a) to (i) of FIG. 7, and shows a normal round dot pattern that is normally used. From FIG. 8A to FIG. 8D, cyan is 15 °, magenta is 45 °, and black is 75 °.
Shows the individual screens of. 8 (e) to 8 (g) show these screens in pairs, that is, superposed in cyan / yellow, magenta / yellow and cyan / magenta pairs. FIG. 8 (h) shows superimposed cyan, magenta and yellow screens. 8 (i) shows the black screen of FIG. 8 (d) overlaid on FIG. 8 (h). Moire effects on both large and small scales are shown in these figures, but they are not so obvious because they are usually in full color image at normal grid spacing or screen lines. . The effect of dot gain is not obvious at all because of the technique these figures must be provided with.

[0027]

Since the present invention is constructed as described above, it is excellent in producing a halftone image having a dot pattern that can be used to reduce the rosette moiré, dot gain and tone jumping effect in a halftone image. Method is provided.

[Brief description of drawings]

FIG. 1 shows a rough regular circle, square and elliptical dot pattern that varies uniformly between light and dark tones.

FIG. 2 shows an image of an illuminated sphere using a regular variable round dot pattern.

FIG. 3 is a diagram showing an inversion state of a rough regular round dot pattern for making a printed area portion having an inwardly curved edge.

FIG. 4 is a diagram showing an inverted state of a variable oval dot pattern in which dots are changed from a circle to an oval shape and vice versa according to the present invention.

FIG. 5 shows an example of contours of variable oval dots superimposed on a common center according to the present invention.

FIG. 6 shows two regular round dot screens superimposed at 30 ° and two inverted dot screens superimposed at 90 ° according to the invention.

FIG. 7 is a diagram showing an example of a dot pattern for performing various screen combinations according to the present invention.

FIG. 8 is a diagram showing a corresponding normal round dot pattern contrasted with FIG. 7.

FIG. 9 is a flow chart showing a general method of practicing the present invention.

[Procedure amendment]

[Submission date] November 18, 1993

[Procedure 3]

[Document name to be corrected] Drawing

[Correction target item name] Figure 9

[Correction method] Change

[Correction content]

[Figure 9]

 ─────────────────────────────────────────────────── ─── Continuation of front page (31) Priority claim number 242583 (32) Priority date May 1, 1992 (33) Priority claiming country New Zealand (NZ)

Claims (27)

[Claims] 1. A method of producing a halftone image having an inwardly curved edge and adapted to produce a change in tone in light tones by creating print dots that are gradually extended as the tone darkens. . 2. The method of claim 1, wherein the dots are stretched in a direction that substantially darkens the tone. 3. The method according to claim 1, wherein the edges of the dots are part of an ellipse. 4. A method according to any of the preceding claims, wherein the dots are stretched along a plane which is substantially mirror image symmetric. 5. A method according to any of the preceding claims, wherein the dots are adapted to have two planes that are just substantially mirror image symmetric. 6. A method according to any of the preceding claims, wherein the dots are adapted to merge into their nearest neighbor in two separate steps as the tone darkens. 7. A method according to any of the preceding claims, wherein the edges of the dots are made into a substantially smooth curve that merges to form a point. 8. A method according to any of the preceding claims, wherein the dots are substantially needle-shaped. 9. A method of making a halftone image, wherein the tone variation in a dark tone is made by unprinted dots having outwardly curved edges and gradually extended as the tone becomes lighter. 10. The method of claim 9, wherein the dots are stretched in a direction that is substantially brighter in tone. 11. The method according to claim 9, wherein the dots are substantially elliptical. 12. A method according to any one of claims 9 to 11, wherein the dots are stretched along a plane which is substantially mirror image symmetric. 13. A method as claimed in any one of claims 9 to 12 in which the dots have two planes that are just substantially mirror image symmetric. 14. The method of claim 9, wherein the dots are adapted to merge into a nearest neighbor in two separate steps as the tone gets brighter.
The method described in section. 15. The method according to claim 9, wherein the edges of the dots are formed into a substantially smooth curve. 16. A tone change from "bright" to "dark" on at least the cyan and magenta screens is made by making a printed area portion having inwardly curved edges, and each primary color screen is A method of printing a color halftone image adapted to be positioned at an angle that is a multiple of 45 ° to any other primary color screen. 17. The method of claim 16 wherein the cyan and magenta print area portions in light tones are made into dots which are adapted to be progressively extended as the tones of these colors darken. 18. The non-printed area of cyan and magenta in the dark tones is made into dots which are adapted to be gradually extended as the tones of these colors become lighter.
The method described in 7. 19. A method according to claim 16, wherein the cyan and magenta screens are arranged to be displaced relative to each other. 20. Any one of claims 16 to 19 wherein the tone variation in all color screens is made by making a print area section having inwardly curved edges. The method described in section. 21. A tone variation on a black screen is made by making a printed area portion having inwardly curved edges.
The method according to any one of claims 6 to 20. 22. A method as claimed in any one of claims 16 to 21, wherein the cyan and magenta screens are arranged substantially 90 ° to each other. 23. A method according to any one of claims 16 to 24, wherein the yellow and black screens are arranged substantially 90 ° to each other. 24. The method of claim 21, wherein the black screen is positioned at an angle that is substantially a multiple of 45 ° with respect to any other screen. 25. The black screen is substantially c relative to the cyan and magenta screens.
25. The method of claim 24 adapted to have a lattice spacing reduced by a factor of os45 [deg.]. 26. A computer algorithm for carrying out the method for producing a halftone image according to any one of the preceding claims. 27. A computer controlled device for implementing the method of producing a halftone image according to any one of the preceding claims.